port 25 anonymous mail through telnet

I have setup an email server by following howtoforge guide "http://www.howtoforge.com/perfect-server-ubuntu-12.04-lts-apache2-bind-dovecot-ispconfig-3". Mail server is working perfectly. But the problem is that when someone telnet through port 25 he can send email to internal users without authentication.

I have setup mail server on domain webmail.biz. anyone can connect to that domain via telnet on port 25 and send mails to email id's on same domain like from info@webmail.biz to admin@webmail.biz. I want to stop this anonymous mail sent through telnet. if i close port 25 i don't receive any mail.

Thanks for the information
But in that way anonymous mails will choke our server. On one of my server i am getting approx 2500 anonymous mails per day. Please suggest me any solution. Can i detect these kind of mails and later on destroy them. Cant we prevent it by using two different servers for sending and receiving with different domain name. If its possible then please give me some kind of hint on how to do this.